1. Field of the Invention
The present invention relates to a PET/MRI device and a PET device, and more particularly to a PET/MRI device that can obtain a PET image and an MRI image almost simultaneously in a short time, and a PET device whose PET detector can be brought close to a measurement object for improved sensitivity.
2. Description of the Related Art
PET/CT devices such as shown in FIGS. 1A and 1B have been in widespread use, in which a CT device 100 and a PET device 200 are combined to provide a functional image of the PET device 200 superposed on a morphological image of the CT device 100 for diagnosis. In the diagrams, 10 denotes a patient or subject (hereinafter, referred to generically as a patient) to be measured (tested); 20 denotes a bed on which the patient 10 is put (lies); 22 denotes a device for moving the bed 20 in a horizontal direction; 102 denotes an X-ray tube which is the X-ray source of the CT device 100; 104 denotes an X-ray detector; and 202 denoted detector rings (hereinafter, also referred to simply as rings) which constitute the PET detector of the PET device 200. The CT exposure is typically several times the PET exposure and is therefore not negligible.
Instead of the CT device, an MRI device which can obtain morphological images without radiation exposure is receiving attention. A PET/MRI device capable of obtaining both PET and MRI images has been under research and development. In particular, there has been developed a PET/MRI device of semiconductor light receiving element type in which all the detector units of the PET device are arranged within the static magnetic field of the MRI, using magnetically insensitive avalanche photodiodes (APDs) or Geiger mode APDs (also referred to as SiPMs) as the light receiving elements, and this PET/MRI device can be applied to those for small animals and for the heads (See the following non-patent and patent documents: Schlyer D et al. “A Simultaneous PET/MRI scanner based on RatCAP in small animals,” IEEE Nuclear Science Symposium Conference Record, Volume: 5, pp. 3256-3259, 2007; Schlemmer H W et al. “Simultaneous MR/PET Imaging of the Human Brain: Feasibility Study,” Radiology, 2008: 248, 1028-1035; Judenhofer M S et al. “Simultaneous PET-MRI: a new approach for functional and morphological imaging,” Net Med 2008, 14(4): 459-65; U.S. Pat. No. 7,626,392 B2; U.S. Patent Application Laid-Open No. 2008/0287772 A1; and U.S. Patent Application Laid-Open No. 2009/0108206 A1).
Given detectors of the same sensitivity, the PET device typically increases in sensitivity as the detectors are located closer to the patient and as the field of view in the direction of the body axis of the patient (referred to as axial field of view) is widened. The axial field of view of the PET device as wide as the effective axial field of view of the MRI device (30 to 40 cm or so), which is determined by the stable area of the static magnetic field, has had the problem of insufficient sensitivity of the PET device, requiring a PET measurement time longer than the MRI measurement time (typically several minutes).